In nuclear reactors intended for the generation of power, nuclear fuel assemblies are customarily of the rod type which are arranged in closely spaced parallel arrays in generally square configurations. An outer channel, usually square, surrounds the generally square configuration of fuel rods to form a fuel assembly. The means by which fuel rods are spaced ordinarily take the form of a spacer grid. An example of a spacer grid is shown and described in U.S. Pat. No. 3,654,077.
A common problem in typical boiling water reactors is that the central region of the fuel assemblies may be undermoderated and overenriched. In order to increase the flow of moderator, usually water, and to improve neutron moderation and economy, an elongated central water channel is provided which forms a centrally disposed path for the flow of moderator/coolant along the length of, but physically separated from, the fuel rods. The central water channel can have any cross-sectional area and/or geometry, positioned centrally and symmetrically within the outer channel, or asymmetrically displaced from the central axis within the outer channel, and can be oriented around its central axis so that its walls which extend the length of the assembly are either parallel or non-parallel to the walls of the outer channel. The central water channel can be a square or circular tube or array of such tubes extending along the length of the fuel assembly. An example of the square central water channel is shown in U.S. Pat. No. 4,913,876. Sufficient liquid coolant is circulated through the central channel or tubes to keep the contained coolant largely or completely in the liquid phase. The presence of liquid as contrasted to gaseous moderator in the central region of the fuel assembly increases the nuclear performance of the assembly by providing a greater number of hydrogen atoms which function, in part, to slow down neutrons and thereby increase the likelihood of further fissions. The moderator coefficient of reactivity, which is the change in reactor reactivity occurring when the moderator density changes, is thus affected by maintaining liquid as contrasted to gaseous moderator in the central region in the assembly.
As is well known, each fuel assembly for a boiling water type of water cooled reactor is typically enclosed by a square outer channel which confines the coolant which enters that fuel assembly to that particular fuel assembly until it exits the assembly at the top of the reactor core. The coolant passing through the fuel assembly consists of a mixture of liquid water and steam. At the bottom entrance of the fuel assembly, the coolant is liquid water having a temperature at/or approximately near its saturation temperature. As coolant flows upward through the assembly, power is transferred by the fuel rods to the coolant, steam is produced, and the fraction of steam in the coolant is increased. At the top of the fuel assembly, the coolant which has been heated by the fuel rods may be primarily steam. As a result of a high volume fraction of steam in the upper region of the reactor core, the upper region of the core becomes undermoderated and overenriched due to the presence of too few hydrogen atoms compared to the number of fissionable uranium or plutonium atoms. As a consequence, less than optimum uranium utilization results. The neutronic efficiency may be improved by decreasing the amount of fuel in the upper region of the core. One way in which this may be accomplished is by reducing the diameter of that portion of one or more of the fuel rods which extend into the upper portion of the core.
Current reactor and fuel assembly designs provide for fuel rods to be loaded during fuel fabrication from the top of the fuel assembly. If conventionally designed fuel assemblies are utilized, and if fuel rods with decreased diameters in the upper region of the core are to be loaded into the assembly, the fuel rods must be inserted into the assembly from the bottom. Subsequent to reactor operation, failed fuel rods must similarly be removed from the bottom of the assembly. A failed fuel assembly would first have to be upended which could cause the relocation of cracked fuel pellet fragments within fuel rods which have not failed. Such movement of cracked fuel fragments in rods which have not failed can cause such rods to sustain subsequent fuel cladding failure. The departure from normal procedures by upending fuel assemblies would add to design complexity of the upper and lower tie plates and the fuel rods, increase the fabrication costs of the fuel assembly, as well as increase the subsequent operational costs and risks associated with failed fuel rod replacement.
U.S. Pat. No. 5,084,237 issued to Patterson et al. on Jan. 28, 1992 which is incorporated by reference, relates to a side insertable spacer designed to permit rapid repair of irradiated fuel assemblies. The side insertable spacer is an example of a device which does not require upending of the fuel assemblies in order to remove the failed fuel rod(s). The side insertable spacer, however, must be used in conjunction with conventional spacers and tie plates.
As is well known, improvements in fuel cycle costs may be achieved by increasing the net amount of fuel in the fuel assembly. Although increasing the diameter of the fuel rods would produce such an increase, it would also result in the concomitant increase in the resistance to coolant flow within the assembly and an increase in pressure drop. Spacer grids also contribute significantly to the resistance to coolant flow. Furthermore, since there are several grid spacers which are located at selected intervals along the length of the fuel assembly, their total contribution to resistance to coolant flow affects the maximum quantity of nuclear fuel that may be utilized in a particular fuel assembly design. It would thus be an advantage over prior art designs if a spacer grid offered lower resistance to coolant flow thereby permitting an increase in fuel rod diameter and, concomitantly, an increase in the total amount of nuclear fuel within the assembly.
Once the maximum quantity of nuclear fuel has been placed within the fuel assembly, further improvements in nuclear reactor operations could be achieved if the amount of power that could be safely produced within the fuel assembly were increased. Since reactor power levels are limited by the amount of coolant flowing through the assembly as well as by local heat transfer conditions present at the surface of the fuel rods, it is highly desireable spacer grid offer as little resistance to coolant flow as is possible. It is well known that heat transfer and therefore power capability is enhanced if a continuous film of water is maintained on the surface of the nuclear fuel rods. It would therefore be an advantage over prior art designs if the spacer grid also aided in, or contributed to, maintaining a water film on the fuel rod surfaces. An example of a spacer/mixing grid which provides for circulation of cooling water about the fuel rods while offering low resistance to flow is found in U.S. Pat. No. 4,726,926 for a Mixing Grid issued to Patterson et al. which is incorporated by reference.
In addition to maintaining a water film on the surface of the fuel rods, it is also desirable to transfer liquid water present on the inner walls of the outer channel and the outer walls of the inner water channel to the surface of the fuel rods. In order to insert the outer channel in its proper position over the bundle of fuel rods, a significant clearance is provided between the outer surface of the spacer grid and the inner surface of the outer channel. This clearance permits significant coolant flow between the inner walls of the outer channel and the outer perimeter of the spacer grid. Such bypass flow is undesirable because it is not as effective in the transfer of the liquid film from the channel walls to the fuel rod surfaces as is flow which passes through the spacer grid. Bypass flow can be decreased by limiting the clearance between the spacer grid and channel walls or by sealing the clearance between the spacer grid and the channel walls. Either approach renders the grid spacer susceptible to damage during the insertion or the removal of the outer channel. Copending application Ser. No. 07/747,088, entitled Boiling Water Reactor Fuel Rod Assembly With Fuel Rod Spacer Arrangement describes a fuel rod spacer arrangement wherein the fuel rods can be easily loaded into the assembly. One design which facilitates the transfer of liquid condensed on the surrounding channel to the fuel rod surfaces while maintaining an adequate clearance between the spacer grid and channel is disclosed in U.S. Pat. No. 4,749,543. However, this design suffers from the limitations that it is complex, and since it permits bypass flow, not all of the liquid is removed from the channel walls. It would therefore be an advantage over prior art devices to more effectively reduce bypass flow and remove virtually all liquid present on the inner wall of the outer channel as well as directing it to the fuel rod surfaces.
Spacer grids, regardless of their design, remove or strip a portion of the liquid water which has condensed on the inner walls of the outer channel and outer wall of the central channel and transfer some of the condensed water to the fuel rod surfaces, thereby increasing the water film thickness on the fuel rods. Spacer grids also function to coalesce small liquid droplets present in the coolant flow into larger droplets and aid in directing greater quantities of such larger liquid droplets to the fuel rod surfaces contributing further to increasing the water film thickness on the fuel rods. It might thus appear that additional spacer grids could be placed at selected points along the length of the assembly to function as a flow stripper and transfer liquid coolant drops to the surface of the fuel rods. However, the use of additional spacer grids results in increases in pressure drop. It would thus be an advantage to provide a low pressure drop spacer grid and to position at least one additional low pressure drop spacer grid to reduce the distance between the spacers in the upper region of the core and enhance the formation of a water film on the fuel rods without increasing the pressure drop across the fuel assembly.
Despite advances in the art of fuel assembly and spacer grid designs, a need exists for a spacer grid which has a low pressure drop, improves local heat transfer and provides for maximum fuel loading, while accommodating changes in the diameter of individual fuel rods along the length of the fuel assembly.